Re: Regarding handling of file renames in Btrfs

[Hans van Kranenburg <hans.van.kranenburg@mendix.com>]

On 09/16/2017 02:40 PM, Martin Raiber wrote:
> Hi,
> 
> On 16.09.2017 14:27 Hans van Kranenburg wrote:
>> On 09/10/2017 01:50 AM, Rohan Kadekodi wrote:
>>> I was trying to understand how file renames are handled in Btrfs. I
>>> read the code documentation, but had a problem understanding a few
>>> things.
>>>
>>> During a file rename, btrfs_commit_transaction() is called which is
>>> because Btrfs has to commit the whole FS before storing the
>>> information related to the new renamed file.
>> Can you point to which lines of code you're looking at?
>>
>>> It has to commit the FS
>>> because a rename first does an unlink, which is not recorded in the
>>> btrfs_rename() transaction and so is not logged in the log tree. Is my
>>> understanding correct? [...]
>> Can you also point to where exactly you see this happening? I'd also
>> like to understand more about this.
>>
>> The whole mail thread following this message continues about what a
>> transaction commit is and does etc, but the above question is never
>> answered I think.
>>
>> And I think it's an interesting question. Is a rename a "heavier"
>> operation relative to other file operations?
>>
> as far as I can see it only uses the log tree in some cases where the
> log tree was already used for the file or the parent directory. The
> cases are documented here
> https://github.com/torvalds/linux/blob/master/fs/btrfs/tree-log.c#L45 .
> So rename isn't much heavier than unlink+create.

Ah. I also see that the difficult situations are about moving a file to
another directory.

So, if I just rename a file in the same directory, that's even simpler.

-- 
Hans van Kranenburg